RECENT GAS-PHASE STUDIES OF
..
INTRAMOLECULAR
HYDROGEN
BONDING
HARALD M0LLENDAL
Department ofChemistry
The University of Oslo
P. O. Box 1033, Blindern
N-0315 Oslo
Norway
ABSTRACT. In this paper recent studies of selected molecules containing intramolecular
hydrogen bonds are reviewed.
1.
Introduction
Structural and confonnational studie's of free molecules containing intramolecular
hydrogen (H) bonds have attracted considerable interest in recent years both byeleetron
diffraetionists and microwave (MW) spectroseopists. The MW studies were reviewed in
1987 by Wilson and Smith[I]. At that time, nearly 100 moleeules falling into this
eategory had been studied by this method[I]. In addition, other methods sueh as electron
diffraetion (ED), IR and NMR speetroseopy as well as ah initio and molecular meehanies
eomputations eontinue to contribute greatly. aur laboratory has a eurrent interest in
intramoleeular H bonding. Some reeent results obtain in our and other laboratories are
diseussed in this article.
2.
The ethylene glycol problem
The eonfonnational properties of ethylene glyeol, HOCHZCHZOH, have been
investigated by a variety of methods. The fIrst gas-phase struetural study of this moleeule
was made as early as 1949 by Bastiansen using the ED method[2]. He eould only deteet
277
J. Laane et al. (eds.), Structures and Conformations of Non-Rigid Molecules, 277-301.
@ 1993 Kluwer Academic Publishers. Printed in the Netherlands.
278
the heavy-atom gauche confonner (or confonners) which he presumed is stabilized by
intemal H bonding. In 1986 Hedberg and co-workers[3] repeated and extended
Bastiansen's ED experiment, but even at a temperature of 460°C they could not detect any
heavy-atom anti confonner. In this case, only a few per cent concentration of anti would
have been detected[3]. It can therefore be concluded that gauche is much more stable than
anti.
gGg'
gGa
Figure 1. The two low-energy forms of ethylene glycol, HOCH2CH20H, that are
stabilized
by a OH
...
O internal
H bond.
As seen in Fig. 1, there are two heavy-atom gauche confonners denoted gGa and gGg'
that each can possess an intramolecular
OH
...
O hydrogen bond. There is no way ED
experlments can differentiate between the two because the electron seattering ability of
hydrogen is too small. The ED experiments[2,3] thus left open the question whether the
heavy-atom gauche fonn was made up of gGa, gGg', or both. However, the MW spectra
would be very different because the gGa and gGg' confonners would possess rather
different dipole moment components along their principal inertial axes. MW spectroscopy
should therefore be ideally suited to settle the question of the confonnational composition
associated with the heavy-atom gauche atomic arrangement, but there were
complications: The fIrst MW study[4] of the parent species, HOCH2æ20H,
revealed a
very rich and complicated spectrum. Ordinary rigid-rotor spectra of gGa and/or gGg'
could impossibly account for the observed spectral richness and intensity. It was
therefore assumed[4] that the complicated spectrum was a consequence oflarge-amplitude
tunnelling of the two hydroxyl groups. Ethylene glycol thus revealed itself not only to
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----
279
pose an interesting conformational question, but to harbour a complicated dynamical
problem as weIl.
The tunnelling problem is thought [4] to arise from the following: Appropriate concerted
rotations around the two C-O bonds will interchange the roles of the hydroxyl groups.
The group to the left in Fig. 1 becomes H acceptor instead of donor, and the one to the
right becomes proton donor instead of acceptor, leaving the overall molecule just as it
was. Individual double minimum potentials will then exist both for gGa and gGg'[4]. aand c-type transitions were predicted[4] to be tunnelling rotational-vibrational transitions,
while the b-type transitions were predicted to be non-tunnelling rotational transitions.
In 1980 Walder et ai [5] investigated the MW spectrum of the OOæ2æ20D
isotopic
species, and they were able to assign the spectrum of the gGa rotamer because the
tunnelling frequency is substantially reduced in this isotopic species as compared with
that of the parent species owing to the fact that by deuteration the masses of the tunnelling
atoms have been doubled. The tunnelling frequency was determined to be about 285
MHz. The a- and c-type transitions were found to be combined tunnelling rotationalvibrational transitions, while the b-type transitions were non-tunnelling, as predicted[4].
No assignments were made for gGg', but it was noted that a large number of intense
transitions were unassigned[5]. These workers[5] were also able to determine the dipole
moment.
Tunnelling is completely quenched if an asymmetrical isotopic substitution is made,
because double minimum potentials no longer exist. Ordinary, much simpler, rigid-rotor
spectra wiIl be observed in such cases. This propeny was exploited by Caminati and
Corbelli[6] who assigned the MW spectra of O-monodeuterated species of the gGa
conformer and performed a new measurement of the dipole moment They[6] also
searched for gGg', but could not find it because the spectrum was rather weak as aresult
of the presence of two conformers and several isotopomers.
Methods other than MW spectroscopy and ED have been exploited to unravel the
conformational composition of this compound In an IR experiment using a lowtemperature matrix Frei et aI[7]concluded that only gGa exists in matrices. Takeuchi and
Tasumi[8], however, claimed that both gGa and gGg' are present in substantial amounts
in low-temperature argon matrices. Ab initio computations at an ever increasing level of
sofistication[7,9-12] kept predicting a small energy difference between the two rotamers.
A new search[13] similar to that of Caminati and Corbelli[6] for gGg' was performed
five years ago. This time HOCH2CD20H which is unsymmetrical with respect to
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--
280
hydroxyl-group tunnelling, was used. HOCH2CI)zOH was chosen because a spectrum
approximately twice as strong and much less crowded than that of the previous
experiment[6], would be observed for this isotopomer[13]. This important simplification
of the spectrum has to do with the different distribution of isotopic species between gGa
and gGg'. This time the MW spectrum of gGg' was successfully assigned.,and found to
be non-tunnelling, as expected[4]. The dipole moment of this rotamer was
determined[13], and the gGg'conformer was found to be only 1.4(4) kl mol-I less stable
than gGa.
The complex tunnelling problem of species that are symmetrical with respect to
interconversion of the hydroxyl groups is still not entirely solved and continues to attract
interest. Christen[14] has now succeeded to assign the spectrum of the parent species of
gGa and confirmed a large number of assignments using MW-MW double resonance
spectroscopy. He has found a tunnelling frequency of about 7 GHz[14]. Coudert[15] has
written a program based on Hougen's procedure[16] for large-amplitude tunnelling
motions. With his program Coudert[15,17] has been able to fit the tunnelling spectrum
assigned by Christen[14] of the parent species, HOæ2æ20H,
and the dideuterated
species assigned by Walder et al[5], DOæ2æ20D,
very satisfactorily.
However, assignment of the tunnelling spectrum predicted to exist for gGg' has so far
not been reported. Likewise, tunnelling in vibrationally excited states such as the first
excited state of the C-C torsional vibration of the gGa conformer remains unsolved. The
dynamical problem will thus present a challenge in coming years.
The remarkable stability of the two gauche rotamers over any anti forms deserves
comment. Not only was anti completely absent at a temperature of 460°C[3], but the
strength also manifests itself in the O-C-C-O dihedral angles which are approximately 6°
smaller in both gGa and gGg' than the ordinary gauche angle of 60°[13]. It is unlikely
that the rather weak H bonds in these two conformers can account for this extraordinary
stability.The so-calledgaUÆ:he
effect[18]whichstabilizesthegauchepositionfor
electronegative substituents probably works in concert with the H bond in this case[13]
augmenting the tendency to prefer the gauche arrangement.
In the last two years, newelaborate ah initio computations have been carried out [19-21]
even at a such a high level of theory as MP3/6-31G**[19]. These computations found an
energy differenee between gGa and gGg' so to speak within the experimental value of
1.4(4) kl mol-I [13] and an energy differenee of 8 -12 kl mol-I between gGa and the most
stable heavy-atom anti conformerdenotedaAa'.
-~-
-
-
---
-~
281
It is of interest to compare the fmdings for ethylene glycol to those of similar molecules.
Ethylenediamine, H2NCH2CH2NH2, which is isoelectronic with ethylene glycol, has
two stable heavy-atom gauche conformers similar to gGa and gGg' of Fig. l [22]. Each of
these conformers is stabilized with a NH
...
N intramolecular
H bond and each of them
displays a tunnelling spectrum which is similar to, but has a much less splitting than that
found for ethylene glycol[22]. Both H atoms of both amino groups must participate in the
tunnelling process. This is presumably one reason why tunnelling frequencies in
ethylenediamine is much less than in ethylene glycol[22].
There is another notable difference between ethylene glycol and ethylenediamine: An ED
study[3] found that the heavy-atom anti conformer(s) is only 1.9[5] kl mol-l less stable
than gauche, which contrasts the situation in ethylene glycol[2,3] where anti could not
been detected in the gas phase[3]. The different stability of the heavy-atom gauche
conformers of ethylene glycol as compared to ethylenediamine is probably the result of
two factors: the H bonds are stronger in the former molecule. In addition, the oxygen
atom is more electronegative than the nitrogen atom and the gauche effect[18] is
consequently larger in ethylene glycol than in ethylenediamine.
Ethanedithiol, HSCH2æ2SH,
has been studied by ED[23,24] and MW
spectroscopy[25]. The ED studies[23,24] found essentially no energy difference between
the heavy-atom gauche and anti conformations. Interestingly, the one gauche conformer
that was assigned by MW[25] did not display tunnelling, although the conformation
assigned by MW[25] is similar to gGa of ethylene glycol[24] (see Fig. I). The lack of
splitting could indicate that a high barrier exists in this molecule. It would be interesting to
see if high-Ievel ah initio computations would predict such aresult. Interestingly, absence
of tunnelling was also seen with catechol (l,2-dihydroxybenzene)[26,27]. The reason for
this absence was presumed to be a rather high barrier to tunnelling[27].
3.
Internal H bonding in 3-butenes
It has long been known that the 1telectrons of double bonds may act as proton acceptors
for intramolecular H bonds in cases where hydroxyl groups are proton donors[28]. This
occurs when the double bond is both in a-[1,28], 13-[1,28]or y-position[28]. The
simplest alcohol where the double bond is in l3-position, is 3-buten-I-ol,
HOCH2CH2CH=æ2.
This compound has been investigated in classical studies[29-33]
in dilute solution using IR spectroscopy. These workers concluded that the compound is
282
stabilized by an internal H bond fonned between the hydroxyl group H atom and the 1t
electtons of the double bond. This finding was confmned by ED[34] and MW[35]
studies, which of course are made in the gas phase. Confonners not stabilized by
inttamolecular H bonding were not seen in the gas-phase studies[34,35], and it was
concluded that such rotamers are at least 3 kJ mol-I less stable than the H-bonded
conformer[35]. This is one indication that the H bond is relatively rather sttong in this
compound. Another indication that the OH
...
1t electton H bond is fairly sttong in this
case is the fact that the CI-C2-C3=C4 dihedral angle is 75(3)° from anti (105° from syn).
NormaIly this angle is about 60°. The approximately 15° increase makes the 1telecttons
come into closer contact with the hydroxyl group H atom thereby maximizing the
OH... 1telectton H bond interaction[35]. However, the H bond is not so sttong that
the O-Cl-C2-C3 dihedral angle is reduced below the nonnal60° from syn. Actually, it is
64(3)°[35].
Less information is available about inttamolecular H bonding interaction of other groups
than hydroxyl such as e.g. amino groups or thiol groups with 1telecttons. However,
MW studies of H2NCH2CH2CH=æ2[36]
and HSæ2CH2CH=CH2[37]
have recently
been performed. In the fonner of these two compounds rotation around the three bonds
N-Cl, Cl-C2, and C2-C3 may produce confonnational isomerism. Consequently, a
large number of rotamers is possible. Five selected conformations of
H2Næ2æ2æ=æ2
are depicted in Fig. 2. ane of the two H atoms of the amino
group may be used for internal H bond fonnation. The two heavy-atom gauche forms
denoted Gauche I and Gauche Il each possesses an intern al NR
...
1t electton
H bond.
Gauche I and Gauche Il differ in the orientation of the amino group.
Interconversion between these two confonners are obtained by rotation of the amino
group 120° and at the same time retaining the H bond. The N-CI-C2-C3 dihedral angle is
60° from syn, and the Cl-C2-C3=C4 dihedral angle is 60° from anti and twisted towards
the amino group. In the three extended confonners of Fig. 2 the The N-CI-C2-C3
dihedral angle is 180°. The Cl-C2-C3=C4 dihedral angle is 60° just as in the gauche
confonners. The position of the amino group is the same in Extended I as in Gauche I; in
Extended Il it has the same position as in Gauche Il, white the amino groups takes the
third possible staggered position in Extended Ill. H bonding is of course impossible for
the extended fonns. MW spectta of the four confonners Gauche I and Il and Extended I
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--
-
--
-
283
Gauche
Extended
I
Gauche Il
I
Extended
Extended
Il
III
Figure 2. The Gauche and Extended confonners ofl-amino-3
-butene,
H2NCH2CH2CH=CH2. The Gauchefonns are stabilized by NH
e e en
---
H bonds.
-
----
284
and Il were assigned[38], while Extended [Il was not found in this crowded spectrum,
although it may very well exist as a stable relatively low-energy fonn of the molecl,lle[36].
Gauche [was found to be the most stable confonner. It was found to be 0.8(3) kJ mol-l
more stable than Gauche Il, 1.9(5) kJ mol-l more stable than Extended [ and 2.1(5) kJ
mol-l more stable than Extended Il. There appears to be nothing "unusual" with the
geometries of all the four assigned fonns.
Another similar example is 3-butene-l-thiol, HSCH2CH2CH=CH2, w,herethree
confonners were found by MW spectroscopy[37]. The Gauche confonner is stabilized by
a SH
...
1t electron
HOCH2CH2CH=CH2
H bond
similar
and to the NH
to its OH
...
...
1t electron
1t electron
counterpart
in
H bond found in Gauche
[ and
Gauche Il in H2NCH2CH2CH=CH2 (Fig. 2). The H-bonded Gauche rotamer was found
to be the most stable fonn of the thiol, while Extended [ and Extended Il were found to be
2.9(5) kJ mol-l and 3.6(6) kJ mol-l, respectively, less stable than the Gauche
confonner[37]. Gauche is transfonned into Extended [ by a 120° rotation about the
C2-C3 axis, while Extended [ is transfonned into Extended Il by another 120° rotation
about the S-Cl axis. The geometries of the three confonners of 3-butene-l-thiol revealed
nothing unusual.
Some selected findings for the three 3-butene derivatives considered in this paragraph is
summarized in Table 1.
TABLE 1. Selected confonnational findings for 3-butene derivatives.
Molecule
~o
=(E°Extended - EOGauclæl/kJmol-l.
HOCH2CH2CH=CH2
>3
H2NCH2CH2CH=CH2
1.9(5)a
H2NCH2CH2CH=CH2
1.3(5)b
HSCH2CH2CH=CHz
2.9(5)C
ap,°Extended 1- EOGauclæ l. bEOExtended 11- EOGauclæ Il. cEOExtended 1- EOGauclæ l
It has been argued[38,1] that a simple rotation around a bond followed by no further
significant relaxations may give a rough estimate of the H bond strength. For example,
Gauche [of l-amino-3-butene is transfonned into Extended [by a 120° rotation (See Fig.
2) around the CI-C2 bond. Likewise, Gauche Il is transfonned into Extended Il by a
corresponding rotation. The energy differences associated with such rotations are
collected in Table l, and they can be taken as a rough estimate of the H bond strength. It
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----
285
can be concluded from this table that the H bond in the alcohol is considerably stronger
than in the thiol. Moreover, the amine and the thiol have roughly the same H bond
Gauche Il
Gauche I
Gauche III
Anti I
Figure 3. Five confomers cyclopropanemethanol.
by a OH
...
pse1Pio1telectron H bond.
Anti Il
Gauche I and Anti I may be stabilized
286
strerigths ofroughly 2 kl mol-l, perhaps with the H bond in the thiol tending to be the
slightly stronger one.
4.
The cyclopropyl group as acceptor for internal H bonds
m studies from the late sixities[39,40] of alcohols have shown that the cyclopropane ring
can act as proton acceptor for intramolecular H bonds. The pseudo 1telectrons present
along the edges of the cyclopropyl ring[41] were thought to be the acceptor[39,40]. Fig.
3 shows five selected forms of cyclopropropanemethanol. In the three gauche rotamers
the O-C1-C2-H chain of atoms are 60° from syn, while this dihedral angle is 180° in the
two anti conformations. Intramolecular H bonding is present in Gauche l and in Anti l,
while this interaction is absent in the three other forms. Only Gauche l was found in the
MW studies[42,43].
Little experimental data conceming the H bond strength in this compound are available.
In order to get an idea about the strength of this interaction, ah initio computations using
the 6-31G* basisset with full geometry optimization were performed in this laboratory
for the five conformations shown in Fig. 3. The computations were carried out utilizing
the Gaussian.program package[44]. The five conformers of Fig. 3 were all found to be
stable, as no imaginary vibrational frequencies were calculated for any of them[45]. The
optimized geometries revealed nothing unexpected. Table 2 lists the energy differences
that were predicted for them.
TABLE 2. Calculated energy differences of selected
conformers of cyclopropanemethanol. 6-31G* basis.
Conformation
L\E0/kJmol-l
Gauche l
O
Gauche Il
5.0
Gauche III
4.7
Anti l
4.4
Anti Il
4.7
Absolute energy of Gauche l: -606310.72 kl mol-l
-
-
-
-~
~
-------
287
The ah initio calculations (Table 2) correctly predicts the H-bonded Gauche l confonner
to be the most stable one, while the other rotamers are predicted to be between 4 and 5 kl
mol-l less stable.
An estimate of the H bond strength can be made in the following manner: Alcohols
generally prefer to have the O-H group anti to the adjacent C-C bond. E. g., anti ethanol
is fouod to be 2.9(2) kl mol-l more stable than gauche[46]. The prediction that Gauche
/l, which has the O-H group anti to CI-C2, is 5 kl mol-l less stable than the H-bonded
Gauche l conformer, make us estimate the H bond strength to be about 8 (2.9+ 5) kl
mol-l in this case. This value is suggested to be typical for cyclopropanols having the
hydroxyl group in a position.
The H bond in a-cyclopropane derivatives is sensitive to substituents as 1cyclopropaneethanol, C3HSCH(OH)CH3, demonstrates. This compound can have two
H-bonded O-C-Cring-H gauche conformers, as illustrated in Fig. 4. Only confonner l
was found experimentally[47], while /l was predicted to be at least 4 kl mol-l less stable.
The destabilisation of /l as compared to l is thought to be caused by steric repulsion
between the methyl group and the cyclopropane ring. This destabilisation must be quite
important since the H bond interaction is estimated by ah initio to be about 4-5 kl mol-l
(see Table 2).
I
Il
Figure 4. The two rotamers of l-cyclopropaneethanol
with O-C-Cring-H gauche atomic
arrangements capahle offorming intramolecular OH. . . psudo-n electron H honds.
- --
-
288
I
Il
Figure 5. H-bonded conformers oftrans-2-cyelopropanemethanol.
The pseudo-n electrons might be influenced by substituents of the cyc1opropane ring. In
trans-2-methylcyc1opropanemethanol shown in Fig. 5 H bonding can be used to test the
impact of the methyl substituent. In conformer l the methyl group is remote from the
pseudo n electrons involved in H bonding, while they are elose to this group in rotamer
Il. Jf the methyl group donates electron density to the pseudo n electrons, which is
expected, Il should become more stable than l. However, a tendency to the opposite was
found. In a MW study[48] l was found to be slightly more stable than Il by a tiny 0.9(6)
kl mol-l. Interestingly, ab initio computations at the 6-310** level[48] also finds l as
the more stable by as little as 0.3 kl mol-l.
There is not much evidence in the literature whether other groups such as amino or thiol
groups may give rise to internal H bonding with pseudo-n electrons of the cyc1opropane
ring. Recently, MW spectra of (aminomethyl)cyc1opropane, C3HSCH2NH2,[49] and
cyc1opropanemethanethiol, C3HSæ2SH,[50] have been reponed. In both these cases the
only conformers identified were indeed stabilized by intramolecular H bonding. The
identified form of cyc1opropanemethanethiol is similar to the H-bonded Gauche l
conformer of cyc1opropanemethanol; see Fig. 3. This rotan)er was found to be at least 3
kl mol-l more stable than any otherrotameric form of the molecule[50].
--------
-
-
---
----
289
I
Il
Figure 6. The H-bonded conformations
C3HsæzNHz
of(aminomethy/)cyclopropane.
may use both the H atoms of the amino group for H bonding. This is
illustrated in Fig. 6./ is transformed into 1/ by a 120° rotation around the C-N bond. The
N-C-C-H chain of atoms is gauche in both / and 1/. In the MW study[49] both these
conformers were found. The energy difference between them was only 0.1(2) kl mol-l,
with conformer / as the slightly more stable. No further rotamers were found, and it was
conc1uded that if such forms exist, they must be at least 3 kl mol-l less stable than / or Il.
An interesting geometrical difference between the two H-bonded conformers were seen
in this compound. The C-C-N angle takes an ordinary value of 110.0(15)° in /. This
angle opens up to 116.0(15)° in 1/. The rest of the structure appears to be normal[49].
In the molecules discussed so far in this section the proton donor has been in the a
position. Much less is known about the donor-acceptor relationship when the donor takes
the 13 position. 2-Cyc1opropaneethanol is the natural starting point for this type of
compounds. Indeed, this molecule was investigated in solution several years ago by
IR[51,4O] and NMR[52] spectroscopy. Joris et a/[51] found no evidenee for
intramolecular H bonding, while Oki et a/[40,52] found that such a conformer coexists
with other rotameric forms. The MW spectrum[53] has now been assigned for the Hbonded conformer (Fig. 7). The MW spectrum is quite weak and this was used to show
that this rotamer makes up 10 - 30% of the total at a temperature of -15°C, and that further
conformers must coexist with the H-bonded one[53]. Ab initio computations at the 631G** level of theory have been made for fOUTselected rotamers, and the H-bonded
~ ---
290
conformer was found to be the most stable one of these[53]. However, this conformer
was calculated to be only slightly more stable than the other three forms for which
computations were perforined. There is thus agreement between the ah initio and MW
Figure 7. H-bonded con/ormer o/2-cycloproaneethanol.
findings. ED work has now just been completed and confirms that 2-cyclopropaneethanol
exists as a conformational mixtures of several rotamers[53] with the H-bonded conformer
(Fig. 7) as the most stable conformer. The conformational difference between 2cyclopropaneethanol and cyclopropanemethanol discussed above presumably means that
the internal H bond is slightly stronger in the latter case.
5.
The oxirane ring as acceptor for intramolecular H bonds
Cyclopropane derivatives have only one acceptor, namely the pseudo 1telectrons along
the edges of the ring. The situation with oxirane derivatives is different. These
compounds have three different acceptor sites. The oxygen atom can utilize its lone-pair
electrons to form H bonds. In addition, the pseudo 1telectrons along one of the C-O
edges perhaps with a little help from the lone-pair electrons.on oxygen, can form a second
type of H bond, white the pseudo 1telectrons along the C-C bond can be involved in a
third kind of H bonding. This situation is illustrated in Fig. 8 in the case of
oxiranemethanol
(glycidol). In the H bond inner conformer OH
- ~---
---
-
--~
... O hydrogen
bonding
291
H band inner
H band auter 1
H band auter 2
Figure 8. Three possible conformers of oxiranemEthanol (glycidol) with intramolecular H
bonds. H bond inner has a OH
have OH
...
pseudo
1t electron
...
O H bond, while H bond outer 1 and H bond outer 2
H bonds.
takes place between the oxygen atom of the ring and the hydroxyl group H atom; in H
bond outer l essentially the pseudo 1telectrons along the C-O edge are acceptor, while the
pseudo 1telectrons along the C-C edge are acceptor in H bond outer 2. The two outer
conformations are thus similar to the H-bonded conformer of cyclopropanemethanol and
similar molecules discusSed above, whereas H bond inner has an internal five-membered
OH
...
O hydrogen bond similar to that found in molecules that possess ether and
alcohol functional groups on adjacent carbon atoms[1].
- ---
292
In a 20 years old IR study by Oki and Murayama[54] of several oxirane derivatives it
was concluded that confonners similar to H bond inner and H bond outer l coexist in
solution. No evidence was found for the stable coexistence of H bond outer 2[54]. The
MW spectrum of glycidol was studied at about the same time by Brooks and Sastry[55]
who assigned the ground vibrational state of the H bond inner confonner. This rotamer
has an internal five-membered H bond of intennediate strength, as the non-bonded
distance between H and O is about 30 pm shoner than the sum of the van der Waals
radii[56] of hydrogen and oxygen.
The MW experiment has been repeated very recently[57]. In addition to confinning the
assignments made by Brooks and Sastry[55] for H bond inner, a "new" confonner, H
bond outer l, was assigned. H bond outer l was found to be only 3.6(4) kl mol-l less
stable than H bond inner. The O-CI-C2-C3 dihedral angle which is 27(3)0 from syn in H
bond inner and swings out to 141(3)0 in H bond outer l. The non-bonded distance
between the H atom of the hydroxyl group and the oxygen atom of the oxirane ring is
slightly longer than the sum of the van der Waals radii of oxygen and hydrogen[56]. It is
therefore better to describe this H bond as one where the pseudo 1telectrons rather than
the oxygen atom lone-pair electrons act as proton acceptor. The experimental findings are
in good agreement with ab initio computations at the 6-31G* leve1of theory[57]. E. g.,
these computations predict the 01-CI-C2-C3 dihedral angle to be 28.20 from syn in H
bond inner, and 141.70 in H bond outer l. The latterrotamer was calculated to be 4.5 kl
mol-l less stable than the fonner. H bond outer 2 was not found experimentally, but is
predicted by ab initio to be 10.3 kl mol-l less stable than H bond inner[57].
Oxiranemethanol has one chiral carbon atom and exists in two enantiomeric forms which
of course are indistinguishable by MW spectroscopy. 1-0xiraneethanol, however, has
two adjacent chiral carbons. Threo and erythro diastereomers will therefore exist.
Moreover, there is one enatiomeric pair for the erythro stereoisomer, and one pair for the
threo isomer. The individual mirror images of each pair cannot of course be distinguished
by MW spectroscopy, just as in the case of glycidol. The confonnations H bond inner,
H bond outer l and H bond outer 2 are also possible for both threo and erythro. In Fig. 9
only the two first-mentioned rotameric fonns are depicted.
Only H bond inner was found in the case of erythro[58]. Ab initio calculations at th~ 631G* level predict H bond outer l to be 11.4 kl mol-l less stable than H bond inner in
the case of erythro.
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----
---
293
Threo
H bond inner
Erythro H bond inner
Threo
Erythro
H bond outer 1
H bond outer 1
Figure 9. The H bond inner and H bond outer 1 conformers of threo- and erythro-loxiraneethanol.
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.
294
Both H bond inner and H bond outer l was found for threo[59]. In this case H bond
inner was found to be 2.8(4) kl mol-l more stable than H bond outer l. Ab initio (631G* leve!) predicted this energy difference as 2.2 kl mol-l[59].
100
~
~
ca
E
8c
~
=ca
c
al
E..
c
ca
F
tt.
F
o
3700
3600
3500
o
3700
31.00
Frequency/cm-t
3600
3500
Frequency/cm-'
31.00
Figure 10. The gas-phase fR spectra ofthreo-(left) and erythro-l-oxiraneethanol(right) in
the O-H stretching region. Further description is given in the text.
The conformational difference found for erythro and threo by MW spectroscopy has a
striking parallel in the JR spectra[59] shown in Fig. 10. Only one absorption maximum is
seen for the OH stretching vibration band in erythro, while two such bands are seen for
threo. The prominent splitting in threo presumably reflects the difference between the H
bonds in H bond inner and H bond ourer l, as discussed above. The smaller splittings
seen in Fig. 10 for both threo and erythro is ascribed to rotational fme structure. Similar
splittings of O-H stretching bands were also seen for severa1 oxirane derivatives by Oki
and Murayama[54) in their solution studies and ascribed to the existence of H bond inner
and H bond ourer l rotamers.
The difference in the conformational make-up between erythro and threo deserves
comment. It can be seen in Fig. 9 that the methyl group is brought into rather close
contact with the oxirane ring in H bond outer l in erythro, whereas such a close contact
does not exist in threo. The resulting crowding is reminiscent of the situation encountered,
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295
H bond inner
H bond outer 1
H bond outer 2
Figure 11. The three possible H-bonded conformations of cis-3-methyloxiranemethanol.
for confonnation II of C3H5CH(OH)CH3[47] (see above), and seems to be of a rather
repulsive nature. Perhaps this is reflected in the ah initio predietions for threo that H bond
outer 1 is 11.4 kJ mol-I less stable than H bond inner[59], and the fact that H bond outer
1 could not be detected by MW spectroscopy[58].
The importanee of methyl group repulsion is also seen in the case of cis-3methyloxiranemethanol (Fig. 11). Only H bond outer 1 was assigned for this
molecule[60]. Owing to the weakness ofthis spectrum it was concluded that this rotamer
makes up 15-35% of the gas at O°C. The ab initio method (6-31G* basis) found H bond
inner to be the most stable fonn of this molecule, with H bond outer 11.0 kJ mol-I less
----
----
296
stable. H bond outer 2 was predicted to be 5.8 kJ mol-l less stable than H bond
inner[60].
Il
I
Figure 12. Possible
H-bonded conformations
of2-oxiraneethanol.
The situation in ~substitued oxiraneethanols is quite different from that in the (1oxiraneethanols discussed above. There are two alternative acceptor sites, the oxygen
atom or the pseudo It electrons along the C-C edge (Fig. 12 ). The MW spectrum has
now been assigned[61] for conformer 1 which has a six-membered OH... O hydrogen
bond. Conformer 1/ where the pseudo It electrons of the ring are acceptor, has not been
found experimentally. 1/ is similar to the most stable rotamer of 2-cyclopropaneethanol
discussed above (Fig. 7). Ab initio computation at the MP2/6-31G* level predicts
conformer 1/ of Fig. 12 to be 9.6 kJ mol-l less stable than 1[61].
Finally, preliminary results for thiiranemethanethiol should be mentioned. This
compound which is the disulphur analog of oxiranemethanol (glycidol) is now being
studied[62]. Ab initio computation at the MP2/6-31G* level of theory predict similar
energies for H bond outer l and H bond outer 2, while H bond inner was calculated to
have a higher energy. The conformational make-up of this disulphur analog of
oxiranemethanol is thus predicted to be strikingly different from that of oxiranemethanol,
as will be remembered from the foregoing discussion.
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297
6.
Conclusions
The examples above show that internal H bonding is important for the confonnational and
structural properties in many different type of compounds. In some cases such as e.g.
ethylene glycol, complex dynamical properties are linked with this interaction.
Intramolecular H bonding is not only restricted to "classical" proton donors such as
alcohols, but amines and thiols can also act as donors with a number of acceptors. The
donor properties of thiols and amines are of course much less pronounced than in the case
of alcohols. Nevertheless, amines and thiols are capable of fonning H bonds even with
such relatively weak acceptors as 1tand pseudo 1telecttons in the same way as alcohols
do.
The oxirane ring fumish three different kind of acceptor sites, the oxygen atom and
pseudo 1telectrons along the C-O and C-C bonds, respectively. Interestingly, alcohols
fonn H bonds with at least two of these sites, viz. the oxygen atom and the pseudo 1t
electrons associated with the C-O bond. It would be interesting to see if the
corresponding amines and thiols parallel the alcohols in this respect. This work wiil
remain for the future.
H bonding is not the only non-bonded effect that detennine the confonnation and
structure of a molecule. Other weak interactions such as the gauche effect[18] and steric
repulsion may in many cases be important, or even more important than H bonding.
Sometimes H bonding works in concert with other effects. Ethylene glycol is one such
example where intramolecular H bonding and the gauche effect together is presumed to
stabilize the gGa and gGg' fonns to aremarkable extent[3].
The hydrogen bonds discussed in this paper are all weak. Some of them c1earlyrepresent
border-line cases. Their bond angles and bond distances are thus found to be nearly the
same as for molecules that do not possess H bonds, as expected.
Classical experimental methods such as ED, MW, IR and NMR speCtI'oscopycontinue to
give reliable infonnation about internal H bonding, but ah initio techniques now seem
most promising, at least for small molecules provided a sufficiently large basis set has
been used in the theoretical pred.ictions.In fact, we have good experiences with high-leve!
(6-310* or bener) computations. Rotational constants seem to be predicted correctly to
within a few percent. Dipole moments are often seen to be within 20% or so of the
experimental values. Energy differences also seen to be well pred.icted in most cases. Ab
initio methods are now readily available for most laboratories. There can be linle doubt
--
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298
that this tool will become increasingly useful in confonnational analysis of small
molecules in the years to come, and it has already taken its place as a major method in this
field.
Aeknowledgements. Mrs. Anne Horn is thanked for drawing the figures. I am most
grateful to Cand. real. K.-M. Marstokk for his skilful construction and maintenance of
our scientific equipment and for interesting discussions during 25 years. The Norwegian
Research Council for Science and the Humanities and the Nansen Foundation of the
Norwegian Academy of Science and Letters are thanked for fmancial support through
many years.
7.
l.
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